Fix qemuMigrationToFile nonull annotation

Fix qemuMigrationToFile nonull annotation

The qemuMigrationToFile method was accidentally annotated for
the 'compressor' parameter to be non-null, instead of the
'path' parameter. Thus GCC with -O2, unhelpfully deleted the
entire 'if (compressor == NULL)' block of code during
optimization. Thus NULL was passed to virCommandNew() with
predictably bad results.

* src/qemu/qemu_migration.h: Fix non-null annotation to be
  against path instead of compressor